1. Field of the Invention
This invention relates to DC-to-DC converters, and more particularly to forward single ended converters which switch at zero voltage.
2. Description of the Prior Art
A DC-to-DC converter is a circuit which converts a direct current voltage at one level to a direct current voltage at another level. If the circuit is based upon switching the input DC current through the primary of a transformer and then rectifying it in the secondary of the transformer, the converter is called a single ended forward converter. A singled ended forward converter is gated by a single switch in series with the primary of the transformer. Energy is transferred forward from the input through the primary winding to the secondary winding of the transformer during the ON period of the switch.
One of the primary design goals in such DC-to-DC converters is to increase the amount of power transferred through the converter. One prior art approach is presented by Fred O. Barthold in "Source Volt-Ampere/Load Volt-Ampere Differential Converter", U.S. Pat. No. 4,734,839.
Common prior art DC-to-DC converter topologies include the buck or forward converter, the buck-boost or flyback converter, and the boost converter which transfer energy from the input to the output during the ON time or OFF time of the main switch. In these circuits a dead time is created during the energy transfer which results in an increased in size of the output filter. Barthold combines the forward and flyback topologies to achieve a continuous transfer of energy from the input to the output of the converter to significantly reduce the size of the output filter.
Another prior art method for increasing the power transfer through the converter is to increase the switching frequency. This results in a reduction of the size of the isolation transformer and the output filter. However, there are upper limits to the operating frequency of prior art buck converters, for example, on account of switching losses in the semiconductor switches utilized in the converters. Switching losses occur when the main semiconductor switch in the buck converter is turned ON and OFF due to the finite switching speed or the time required for the current in the semiconductor device to start and stop flowing.
In order to overcome limitations in switching speeds, the prior art has devised a new family of resonant and quasi-resonant DC-to-DC converters. In the case of quasi-resonant converters, the prior art technique consists of shaping the current or voltage to become half sinusoidal and to perform the switching when the current or voltage reaches zero. The reactive elements which contribute to shaping the current or voltage are part of the basic circuit and are considered undesirable in classic topologies because of leakage inductance and parasitic capacitance. An example of one such circuit can be found in Vinciarelli, "Forward Converter Switching at Zero Current", U.S. Pat. No. 4,415,959. The technique utilized by Vinciarelli consists of adding a resonant capacitor across the flywheeling diode to create a resonant circuit in combination with the leakage inductance of the transformer. During the ON time of the main switch, a current charges the resonant capacitor. When the current reaches zero, the main switch turns OFF in the primary of the transformer. The output inductor discharges the resonsant capacitor, transferring the energy to the load. This topology eliminates switching losses which allows the converter to run at a very high frequency. However, this topology exhibits several drawbacks which limit its utilization at low and medium power levels. These drawbacks are as follows.
The peak current in such a quasi-resonant converter is significantly higher than in a conventional forward converter. The peak current becomes substantially large if a large input voltage range is required. The energy in the prior art device is transferred in stages from the input to the resonant capacitor and then from the resonant capacitor to the output. Due to the fact that the switching in the primary occurs at zero current and nonzero voltage, the energy contained in the output capacitance of the main switch is dissipated when it turns ON. Output power is varied by varying the frequency. A certain amount of energy is transferred from the input to the output at every cycle and when the power requirements are high, the repetition frequency is correspondingly high. Modulation of the frequency does not allow significant decrease of the output filter size. A large electromagnetic interference (EMI) filter is required to avoid beat frequency problems between the units, if two non-synchronized units are used together.
Another family of quasi-resonant converters which switch at zero voltage is described by F. C. Lee, "Pulse Width Modulation Technique", High Frequency Power Conversion International Proceedings (April 1987), Intertec Communications, Ventura, California. These prior art circuits operate similarly to those described above with the exception that the main switch turns ON and OFF at zero voltage. This has the advantage of eliminating the losses caused by the discharge of the capacitance of the switch at turn on and also decreases the driving current utilized in the MOSFET switch due to elimination of the Miller effect, i.e. induced increase in capacitance due to anode to cathode charging.
However, the voltage stress across the main switch and the frequency modulation which is required make this topology unattractive.
An additional group of quasi-resonant converters includes the multi-resonant converters such as were described at the High Frequency Power Conversion International Proceedings (May, 1988), Intertec Communications, Ventura, California. While operating similarly to other quasi-resonant topologies, a secondary resonant circuit is employed to decrease the stress across the output rectifier and to reduce frequency swings over various input-output conditions of operation.
What is needed is a converter which can operate constant frequency yet eliminates current or voltage stresses which are characteristic of prior art quasi-resonant converters while at the same time shaping the voltage or current only at the switching time.